Cancer incidence and mortality rates may be used to assess consistency with hypotheses regarding cancer etiology suggested by other scientific studies. An analysis of the potential effect of SV-40 contamination of poliovirus vaccine found that after more than 30 years of follow-up, no association was found with significantly increased rates of medulloblastoma in the United States. In response to concern that environmental factors may be increasing childhood cancer incidence rates, our analysis found no substantial changes in incidence for the major pediatric cancers, and rates have remained relative stable since the mid-1980s. Ongoing analyses are investigating the relation between ovarian cancer mortality trends and parity and oral contraceptive use, which affect risk and have changed over time in the United States. We are evaluating the incidence patterns of non-Hodgkins lymphoma by histologic type to identify categories that descriptively appear similar and thus may etiologically similar, versus categories that look dissimilar and thus may be etiologically distinct. As we analyze the geographic gradients in melanoma mortality, we are evaluating the potential effect of ultraviolet radiation on rates among blacks as well as whites and whether the latitudinal and radiation effects may have changed over time. In response to a report relating ultraviolet radiation and retinoblastoma, we are evaluating the relation within the United States and internationally, controlling for such potential confounders as race, economic development, and geographic region.